A sustainable solution for aquaculture wastewater treatment: Evaluation of tannin-based and conventional coagulants

The global population faces increasing challenges in securing fresh water and food production. Aquaculture, a key source for obtaining protein, contributes significantly to environmental pollution, releasing suspended solids, organic matter, nitrogen, and phosphorus into water bodies. Some wastewater treatment methods can be expensive due to high equipment, energy, reagents, and maintenance costs, prompting the search for more sustainable alternatives. This study evaluates the performance of tannin-based coagulants from Castanea sativa shells (CS) for treating the recirculating water in an aquaculture plant. Two tannin-based coagulants, C1 and C2, were produced using ethanolamine (ETA) and NH₄Cl, respectively. The efficiency in removing color, turbidity, organic matter, nitrogen, and phosphorus was compared to a commercial tannin-based coagulant (Tanfloc SG) and chemical ones (FeCl₃ and Al₂(SO₄)₃). Coagulant C2, at 10 mg L^-1, removed 90 % of turbidity and 32 % of phosphorus, outperforming the Al-based coagulants (20 mg L^-1). Moreover, CS-based coagulants reduced natural organic matter (NOM), expressed as UV absorbance at 254 nm, and dissolved organic carbon (DOC), though none significantly reduced the nitrogen levels. Toxicity tests indicated low bacterial inhibition, with C1 and C2 showing inhibition rates of 8-10 %. The sludge produced was rich in calcium (97-1500 mg g^-1), magnesium (408-638 mg g^-1), and potassium (347-894 mg g^-1), making it suitable as a soil conditioner, though further research is necessary. Tannin-based coagulants, particularly from chestnut shells, offer a promising, sustainable solution for the post-treatment of the effluent from an aquaponics system on a fish farm while reducing organic sludge production.